1. Field of the Invention
The invention described in this patent pertains to the polishing and planarization of integrated circuit surfaces, particularly those comprising a metal, a barrier layer, and an insulating layer.
2. Discussion of Related Art
Chemical/Mechanical Planarization (or polishing), or CMP, is an enabling technology used in the semiconductor industry to remove/planarize various thin films from the surface of semiconductor substrates during the production of integrated circuits. While initial applications of this technology focused on the polishing of dielectric films (such as SiO2), polishing of metal films used for circuit interconnects is undergoing rapid growth. Currently, tungsten and aluminum are the most common metals used for interconnect structures. However, copper interconnects, coupled with low-k dielectrics, have the potential (when compared to Al/SiO2) to increase chip speed, reduce the number of metal layers required, minimize power dissipation, and reduce manufacturing costs.
However, the challenges associated with the successful integration of copper interconnects are not trivial. A typical copper interconnect structure contains a trench formed in silicon dioxide (typically 10,000 angstroms deep and 1-100 microns wide) formed above the silicon substrate. A barrier layer of material (used to improve adhesion of the copper as well as inhibit the diffusion of copper into the dielectric structure) is typically deposited after the trench is formed, and is usually composed of either tantalum, tantalum nitride, titanium, or titanium nitride. This barrier material is also deposited on the horizontal dielectric surface above the trench. The barrier layer is typically  less than 1000 angstroms thick. Copper is then deposited by chemical vapor deposition or electroplating on top of this structure in order to fill the trench structure. To insure complete filling of the trench, an overlayer of copper of 10,000-15,000 angstroms is usually required. CMP is then used to remove the overburden of copper above the trench and the horizontal barrier material above the trench. In order to do this successfully and economically, the copper removal should be as fast as possible, typically above 3000 angstroms/minute. Also, to avoid removal of the copper within the trench (typically referred to as xe2x80x9cdishingxe2x80x9d), removal of the barrier layer at rates comparable to that of the copper film are necessary. Additionally, to avoid degradation of the SiO2 film beneath the barrier layer (typically referred to as xe2x80x9cerosionxe2x80x9d), and to improve global planarization, the removal rate of the underlying dielectric film should be as low as possible. In summary, the selectivity for the removal rate of the barrier film (tantalum, tantalum nitride, titanium, or titanium nitride) should be high with respect to the copper film, while the selectivity for the removal rate of the dielectric film (SiO2) should be low (preferably  less than 100:1).
To accomplish these requirements, a two-step polishing process using two different slurries has been proposed. In U.S. Pat. No. 5,676,587, Selective Polish Process for Titanium, Titanium Nitride, Tantalum, and Tantalum Nitride, a two-step process using first (1) a slurry to remove the majority of the metal film (such as tungsten or copper) and second (2) a slurry to remove the barrier film is proposed.
To suppress the removal rate of silicon dioxide during CMP processes, various additives have been previously suggested that passivate the silicon dioxide surface. In U.S. Pat. No. 5,614,444, Method of using Additives with Silica-Based slurries to Enhance Selectivity in Metal CMP, an additive comprising at least one polar component and one apolar component is suggested to suppress oxide removal. This patent lists a number of compounds containing both polar and non-polar groups, which are either anionic (potassium butylsulphate), cationic (tetrabutyl ammonium hydroxide), or non-ionic (butanol). However, this patent claims as a necessity both a polar and apolar component (group) to be present.
U.S. Pat. No. 5,876,490 polyelectrolytes are used to coat the abrasive particles in a slurry. The polyelectrolytes impart normal stress effects to the slurries. In solution, the polyelectrolytes exhibit normal stress effects and their adsorption on the abrasive particles impart the same behavior to the particles.
According to U.S. Pat. No. 5,876,490, in order to achieve planarization, the quantity of polyelectrolyte in the abrasive suspension is such that a fraction of the particles will be coated with the polyelectrolyte, while another fraction of the abrasive particles will remain uncoated. In order to achieve this, the weight percent of the polyelectrolyte should be about 5 to about 50 percent, preferably about 15 to about 30 percent by, weight, and most preferably about 20 percent by weight of the abrasive particles in the slurry. These ratios depend somewhat on the relative size of the abrasive particles and the polyelectrolyte.
The slurry compositions of U.S. Pat. No. 5,876,490 that contain the polyelectrolyte are preferably prepared by adding the polyelectrolyte to the slurry already containing the abrasive particles, thereby coating a fraction of the abrasive particles xe2x80x9cin situ.xe2x80x9d In an alternative procedure, a fraction of the abrasive particles can be precoated and then admixed with the slurry containing the remaining abrasive particles which will be uncoated. In addition, it may be desirable to pretreat a portion of the abrasive particles to render them more susceptible to adsorption of the polyelectrolyte from the slurry.
U.S. Pat. Nos. 5,391,258; 5,476,606; 5,738,800; 5,770,103 describe compounds which in CMP slurries provide attenuation of silicon dioxide removal. These patents are hereby incorporated by reference and made a part of this specification.
The present invention is directed to one or more organic polymers which have surprisingly been found to attenuate the removal of the oxide film during metal CMP and offers an improvement over earlier slurries. This organic polymer is a high molecular weight organic polymer containing a carbon backbone with functional moieties extending from the backbone. The functional moieties interact strongly with the silicon dioxide surface so as to provide a protective layer that inhibits the removal of the silicon dioxide film at appreciable levels. The mechanism of interaction between the functional moieties and the hydroxyl surface is, though not limited to, that observed in the hydrogen bonding of polar species (such as the interaction of hydroxyl groups). The organic polymer is further defined as a high molecular weight organic material, having a degree of polymerization of at least 3 (i.e., 3 monomeric units polymerized into a molecule), more preferably more than 10, and most preferably greater than 50. The organic polymer comprises a plurality of moieties having affinity to surface groups (i.e., silanol and siloxane) contained on silicon dioxide surfaces. These moieties are commonly polar moieties, such as, but not limited to, hydroxy, carboxy, carbonyl, alkoxy, sulphonyl, and phosphonyl. Examples of this type of organic polymer molecule includes polyvinyl alcohol, polyvinylpyrrolidone, polymethylmethacrylate, polyformaldehyde, polyethylene oxide, polyethylene glycol, and polymethacrylic acid.
Many of these same compounds are mentioned as being useful for coating abrasive particles in U.S. Pat. No. 5,876,490 as discussed above. Their use as a silicon dioxide rate suppressant is not mentioned in ""490. Moreover, the polyelectrolytes of the present invention have been found to be effective as a silicon dioxide rate suppressant at concentrations below about 5 percent by weight of the abrasive particles in a slurry. They have also been found to be effective when having a molecular weight of greater than about 10,000.
Another aspect of the present invention is a method of polishing a substrate comprising a metal and an insulator wherein the substrate is pressed against a polishing pad, the substrate and the pad are moved relative to each other, and a polishing composition is applied to said pad during the polishing operation. The polishing compositions of the present invention are useful for such methods.
The slurries used in this invention were prepared with the following general protocol. In every case, the chemical additives are first dissolved in deionized water. After all the chemical additives are dissolved in the deionized water, the pH is adjusted to the desired level. In a separate vessel, the abrasive package which is comprised of the inorganic oxide abrasive particles in deionized water is mixed. The pH of the abrasive package is also adjusted to the desired level. The final step in the slurry formulation preparation is the combining of the aqueous chemical package with the aqueous abrasive package. Contrary to prior art, a polyelectrolyte additive can be added into this aqueous solution without any special abrasive adsorption requirements.
Typically, the list of chemical additives includes an oxidizing agent, the organic polymer removal rate suppressant of this invention, and optionally a complexing agent and/or a dispersant. The order of mixing of this chemical package need only be chosen such that there is complete solubilization of all the additives.
A complex as defined in xe2x80x9cAdvanced Inorganic Chemistryxe2x80x9d, F. A. Cotton and G. Wilkinson, 3rd ed., Wiley Interscience is: xe2x80x9cThe terms xe2x80x98coordination compoundxe2x80x99 and xe2x80x98complexxe2x80x99 may be broadly defined to embrace all species, charged or uncharged, in which a central atom is surrounded by a set of outer or ligand atoms, whereby the energy of the system is lowered. (i.e. E greater than 0 and/or G less than 0). An example of a neutral complex is SF6, where the central S atom is surrounded by 6 F atoms in an octahedral arrangement. An example of a positive complex ion is [Cu(NH3)4]2+, where the central Cu atom is surrounded by 4 NH3 molecules in a tetrahedral arrangement. An example of a negative complex ion is [Cu(Cl)5]3xe2x88x92, where the central Cu atom is surrounded by 5 Cl atoms in a pentagonal bipyramid arrangement.xe2x80x9d Examples of common ligands, which in the slurries of this invention are called complexing agents, are acetic acid, citric acid, ethyl acetoacetate, glycolic acid, glyoxylic acid, lactic acid, malic acid, oxalic acid, salicylic acid, sodium diethyldithiocarbamate, succinic acid, tartaric acid, thioglycolic acid, glycine, alanine, aspartic acid, ethylene diamine, trimethylene diamine, 1,2 ethanedithiol, 1,4 dithiothreitol, bis(methylthio)methane, dimethyldithiocarbamate, 5-methyl 3,4 thiadiazole-2-thiol, malonic acid, gluteric acid, 3-hydroxybutyric acid, proprionic acid, pthallic acid, isopthallic acid, 3-hydroxy salicylic acid, 3,5-dihydroxy salicylic acid, and galic acid.
The slurries of this invention may optionally comprise a dispersant. Aqueous CMP slurries contain submicron abrasive particles. The size of these particles is important to the performance of the slurry as well as to the resultant surface quality. If the abrasive particles agglomerate, the polishing removal rates may change and the surface quality may deteriorate. Dispersants can be included in the slurry formulation to prevent this agglomeration of abrasive particles. Dispersants can be anionic, cationic, or nonionic. The selection of the proper dispersant depends on many factors including the surface characteristics of the abrasive particles and the ionic nature of the slurry formulation. Some examples of ionic surfactants include sodium lauryl sulfate, cetyl-trimethyl ammonium bromide.
The oxidizing agent in the compositions of the present invention may be comprised of any of the common oxidizing agents such as nitrates, iodates, chlorates, perchlorates, chlorites, sulphates, persulphates, peroxides, ozonated water, and oxygenated water. Oxidizing agents can be used in slurries for CMP at concentrations of about 0.01% to about 7% by weight. Generally they are used at concentrations of about 1% to about 7% by weight. An iodate is a preferred oxidizing agent. Most preferred is potassium iodate at about 2% to about 4% by weight.
In the examples presented below, silica and titania were predominantly used as the abrasive component in the slurries tested. However, any metal oxide or polishing abrasive (such as alumina, ceria, zirconia, barium carbonate, or diamond) could also be used.